2000 jeep grand cherokee headlights 10pts?

[AcademyBound]

So i own a 2000 jeep cherokee and before i purchased it the front headlights of the vehicle were all dirty looking and really distorted the light coming from the light bulb of the actual headlight, in other words the plastic that covers the headlamp headlights was grimmy looking. I have recently purchased new headlamp headlights for my jeep , but before i put them in i would like to know the cause of what made the ones prior look al gross, old and grimmy ? ( i was told maybe from someone waxing them ? ) 10 points for best answer